1. Field of the Invention
In the co-pending, commonly assigned application Ser. No. 09/244,680, an apparatus for measuring characteristics of pulsed light beams, primarily lasers, has been disclosed. The characteristics which are measured by the apparatus include phase, intensity, beam quality, M2 factor, RMS wavefront error and other beam parameters. In the field of lasers and their applications, these parameters are of particular utility and interest.
The present invention is directed to use of pulsed wavefront sensors for applications in addition to measurement of pulsed lasers. In particular, the present invention is directed to using a pulsed wavefront sensor to measure moving elements, to simplify measurements involving moving parts, and to reduce exposure, particularly for use with biological systems.
2. Description of Related Art
In the optical metrology art, a surface to be measured, e.g., lens, mirror, wafer, metal of plastic film, disk platen, etc., is used as a reflector for a light beam. This reflected beam is measured with an optical sensor that measures the wavefront or phase of the beam. The phase measurement translates to a measurement to the surface of interest through one or more simple operations. While there are other, non-optical techniques for measuring surfaces, these do not have the particular advantages of speed, accuracy, dynamic range that will become apparent in this description. However, due to the requirement that other sources of relative motion, such as vibration, be eliminated between successive measurements, it is difficult to use optical metrology to measure characteristics of either moving objects or objects requiring relative motion between a sensor and the object for measurement thereof.
One example of optical metrology is the interferometric measurement of computer hard disk platens. For proper operation, the platen is to be as flat as possible to avoid collisions of the disk head with the surface of the platen. To test the platen, the disk platen is mounted in front of an interferometer or other wavefront or phase measuring device, and the characteristics of the light reflected therefrom are determined. For the case of an interferometer, this measurement requires four to six frames of images, where a reference mirror has been moved between images to provide a known phase shift. The data can then be interpreted and analyzed to produce the surface shape. Using these techniques, accuracy down to a few nanometers can be obtained.
However, in order to make accurate measurements, the disk platen must be held absolutely still between successive images. While this is possible using a vibration isolated platform for both the measuring instrument and the surface of interest, this stability requirement complicates, increases cost, and reduces the tolerance of the instrument to environmental effects. While there are other devices that can measure the wavefront or phase with fewer frames, these devices still have a finite measurement time.
If only the static deformations of the disk are of interest, then the interferometric techniques may be adequate. However, computer hard disk platens are spun at a high revolution rate. This can induce vibrations, modes, and surface shape changes that are only present during operation. With an interferometer or other prior art, these deformations can only be measured in an average sense.
Another example of measuring a moving object is the case of measuring an object that is larger than the aperture of the sensor. More details of this application are presented in co-pending, commonly assigned U.S. application Ser. No. 09/340,502 entitled xe2x80x9cApparatus and Method for Evaluating a Target Larger than a Measuring Aperture of a Sensorxe2x80x9d, which is hereby incorporated by reference in its entirety for all purposes. In this application, a large diameter mirror, wafer or other object is to be measured by a series of possibly overlapping measurements of the wavefront sensor. In this application, the position of the object or sensor is moved from one location to another on some form of motor controlled stage or scanning system. To measure the whole surface, many different positions of the stage must be realized, and an accurate measurement made in each case. The sequence of measurements is then pieced back together to form an overall measurement of the whole object.
A potential problem with this technique is that an accurate measurement must be made for each position of the object relative to the sensor. Thus the object must be moved to a new position, vibrations and oscillations must be damped out to an acceptable level, and then the wavefront sensor may acquire an image for analysis. For a mechanical stage (for example Dynamic Automated Systems model DAS AMB-300), this may take 100-300 ms for each new position, even when adjacent measurements are only 10 mm apart. This limits the total throughput of the measurement process, with the settling time being the dominant component.
Another use for wavefront measurement is in ophthalmic measurement. In ophthalmic measurement, a beam of light is projected into an eye to form a small spot on the retina. This spot is observed through an optical system to provide appropriate information for the wavefront sensor. In this type of measurement, it is desirable to minimize the total optical exposure of the eye and retina in order to avoid damage or discomfort to the patient under study. Thus the total exposure time should be kept to a minimum, as well as the total amount of energy delivered. The problem is that the retina is a fairly poor reflector of light. Furthermore, when using a Shack-Hartmann wavefront sensor, the return light is divided among a large number of focal spots for the. To improve the resolution of the measurement, it is desirable to divide the light among more focal spots. This leads to the need for more and more light to be projected to and received from the retina.
One characteristic of many modern CCD cameras, of particular concern when used for biological measurements, such as ophthalmic measurement, is that these cameras often have a separate exposure and read out time. During readout, the CCD element is not sensitive to light. Hence, CCD cameras they have a duty cycle that is less than 100%. A typical progressive scan CCD camera will have 5-50% duty cycle. This means that subject, e.g., an eye, is receiving exposure that is not necessary or useful for the measurement.
The present invention is therefore directed to using a wavefront sensor which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
It is an object of the present invention is to perform metrology using short, temporally resolved measurements in order to xe2x80x9cfreezexe2x80x9d the deformation of the object at a particular instant in time.
It is a further object of the present invention to perform metrology of even an extremely rapidly moving object.
It is a further object of the present invention to reduce the total exposure of an object while increasing the amount of light available for the measurement.
The above and other objects of the present invention may be realized by using a light source that may be pulsed or controlled temporally and a wavefront sensor capable of detecting such a light beam after having interacted with an object, particularly an object moving relative to the sensor. This combination can be used to optically measure a system, object or combination of elements in a temporally resolved fashion.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.